This has happened to me once when the car was parked in a parking lot. My conclusion:

Do you have comfort access? If so, and the keys are in your pocket with something else in it or just bending down, you may have accidently tripped the 'open hatch' button. I am now more careful in not having anything else in the pocket that I carry the key in.

Just an idea.

Same thing has happened to me twice with an F10 but I had nothing else in my jeans pocket. The remote has far too long a range, 50+ feet even through walls. Lucky for me both times it was viewed by me or someone who knew me. First time was in my mom's driveway and I was seated in the kitchen with my back turned. Other time I was seated in my doctor's office and just happened to be looking out the window. Had this happened in a parking lot with my back turned, who knows what might be missing.